Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 171100
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
400 AM MST Sat Feb 17 2018

.SYNOPSIS...A mild day is expected across the area today. A
significant winter storm will move slowly through the region
Sunday through Tuesday.

&&

.SHORT TERM (THROUGH 12Z TUESDAY)...
Antecedent conditions:
Severe Drought continues along and east of I-15 south of Salt Lake
City, with most other areas in Moderate Drought. While abnormally
dry, the Idaho border area is doing much better in the
precipitation department.

Big Picture:
During the past 5 days, 500mb height and anomaly shows an
unorganized ridge off the West Coast allowing several weak
disturbances and troughs to impact the area. However now with
significant lowering of heights over the western Pacific, seeing
amplification which is reinforcing the ridge over the eastern
Pacific.

Water Vapor loop shows all systems steering around the
aforementioned ridge. One departed the Northern Rockies overnight,
with the next pair over the British Columbia coast and the
southern Alaska coast.

400-200mb MDCARS wind observations place a strong 180-210kt
westerly jet crossing the western Pacific, with a 165-195kt
southwesterly jet from the Southwest Conus through the Great Lakes
region. Over the eastern Pacific, a 140-165kt anticyclonic jet has
rounded the ridge and is nosing into the Pacific Northwest.

The large scale flow is supporting an Atmospheric River around the
ridge into the Pacific Northwest. Integrated Water Vapor Transport
is currently maximized over the Pacific Northwest.

Local Trends and Observations:
24 hour surface observation trends show temperatures are 5F to 15F
warmer and dewpoints are 5F to 10F higher.

Strong high pressure has weakened over the region, due to wave
passing to our northwest. That supported an increase in wind gusts
along the ridges and most locations near Wyoming. Precipitable
water values range from 0.05"-0.10" mountains to 0.15"-0.30" valleys.

Forecast:
The next pair of waves and aforementioned upper level jet continue
to round the ridge and will propagate southeast from British
Columbia toward our region. Valleys look to near Winter Storm
Warning criteria in respect to forecast snow on sufficiently cold
surfaces. Lake enhancement may prolong the event in areas. The
next colder and moist storm system that propagates toward the
region from British Columbia aids in the development of a cold
trough just to our west Sunday into Monday. This will lead to
strong height falls beginning Saturday night, maximizing by Sunday
night. This evolving storm will force a warm front through the
region from southwest to northeast today, so have continued low
PoPs northeast of Salt Lake City where isentropic surfaces support
best lift and saturation.

Global models in good agreement for most large scale parameters
with the trough, with the first one carving out the trough
followed by the second wave reinforcing the trough and
maintaining its position to our west for a significant amount of
time.

We appear to be in a very favorable position east of the slowly
developing trough, with the left exit region of the upper level
jet overhead (and above the cold front), along with a prolonged
period of Positive Vorticity Advection, diffluence aloft and cold
advection, as well as increase of Integrated Water Vapor
transport late tonight and tomorrow. Northwesterly orographic
flow will be in play, along with the possibility of lake
enhancement during the second half of the storm. This is the type
of storm that over-produces expectations (so long as the models
maintain current track). Models have trended with better dynamics
including south central Utah so we have extended the Winter Storm
Watch south to include Cedar City and Richfield.

Cold front makes its way from far northwest Utah Sunday morning to
south of I-80 by Sunday evening, before clearing the remainder of
the forecast area except southeast areas by Monday morning.
Banding and higher precipitation rates associated with the cold
front looks likely given Frontogenesis/Fn convergence within a
saturated environment supportive of negative Equivalent Potential
Vorticity.

GFS time series at SLC shows strong and deep lift late tonight
through Sunday evening. Dendritic crosshair of omega/RH and -12F
to -18F temperatures is seen during this period, with models
trending with stronger lift higher into the Dendritic temperature
zone for SLC, a very encouraging sign. Suspect benches will see
full effect of Dendritic Snow Growth.

Potential Impacts:
- Strong ridgetop winds today due to passage of wave to our
  northeast last night
- Warm today ahead of the storm, about 10F-15F above normal but
  records look safe
- Pre-frontal winds look quite gusty tonight and Sunday south of
  the front. Wind advisories/warnings may be needed for some
  valleys. Ridgetop winds near 70kt likely ahead of the cold
  front.
- Most significant snow seen for valleys in quite a while tomorrow
  into early next week. Road impact timing depends how much these
  cold roads warm today before the system arrives, though high
  precipitation rates would likely overcome warm roads anyway.
  Many northern and western valleys look to reach Winter Storm
  Warning criteria in respect to forecast snow on sufficiently
  cold surfaces, with even higher totals on the benches. Lake
  enhancement may prolong the event in areas.

.LONG TERM (AFTER 12Z TUESDAY)...
The cold airmass and moist northwesterly flow remain over the
area heading into Tuesday, likely allowing snow showers to
continue, particularly along the higher terrain. However, these
showers should taper off relatively quickly through the day, as
the trough axis shifts east of the area, leading to a drying and
stabilizing airmass.

Skies currently look to clear by 06-12Z Wednesday, which would lead
to one of the coldest mornings in quite a while. Global models begin
to diverge Wednesday, but in general Wednesday and Thursday look to
be fairly dry, with the airmass warming a bit, but temperatures
still remaining below seasonal normals for once.

Details differ significantly heading into the end of the week and
the weekend, but the general pattern remains the same, with a series
of shortwave troughs likely to dive through the Great Basin from the
northwest. This could bring another shot of precipitation to the
area Friday and/or Saturday.

&&

.AVIATION...
Winds at the SLC terminal are expected to remain out of
the south through the day, but there is a 30 percent chance that
winds switch to the northwest between 19Z and 22Z. There is also a
slight chance of brief ceilings below 7000 feet between 11Z and 15Z,
but in general the lower clouds should dissipate through the
morning.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Storm Watch from Sunday afternoon through Tuesday morning
     for UTZ003-004-006-008>010-014>016-517.

     Winter Storm Watch from late tonight through Tuesday morning for
     UTZ001-002-005-007.

WY...Winter Storm Watch from late tonight through Tuesday morning for
     WYZ021.

&&

$$

SHORT TERM...10
LONG TERM/AVIATION...Schoening

For more information from NOAA`s National Weather Service visit...
http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php



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